1. Field of the Invention
The invention relates to a heating apparatus using a ceramic heater.
2. Description of the Related Art
The semiconductor-producing apparatus is equipped with a ceramic heater to heat wafers, which are substrates, for manufacturing semiconductor films from raw material gases such as silane gas by thermal CVD or the like.
The configuration so-called a dual zone heater is known as a ceramic heater. For the dual zone heater, an inner resistance-heating element and an outer resistance-heating element comprising a high-melting point metal such as molybdenum are buried in a ceramic substrate. Separate current-introducing terminals are connected to the respective resistance-heating elements, and the inner resistance-heating element and the outer heating element are independently controlled by applying a predetermined voltage to each resistance-heating element.
Further, JP-5-326112,A discloses that resistance-heating elements of a ceramic heater are constituted of circuit patterns comprising a plurality of high-melting point metals or the like, wherein the circuit patterns are arranged to compensate their deficient areas with each other. Specifically, one circuit pattern overlaps the deficient areas such as fold or turnover portion of other circuit pattern.
Particularly for an application to heating semiconductor wafers, controlling the temperature throughout the heating surface uniformly is necessary, and satisfying strict specifications such as temperature differences being within xc2x15xc2x0 C. throughout the heating surface under a use condition is required. A ceramic heater of dual zone controlling system can usually satisfy such specifications. However, the inventors further studied and found that even the ceramic heater, which kept the temperature uniformly within a range from room temperature to 500xc2x0 C. throughout the heating surface, suddenly caused large temperature differences on the heating surface in a temperature range of, for example, 600xc2x0 C. or higher. In such a case, heat spots usually occurred at particular parts on a peripheral portion of a substrate of the heater. At the same time, the resistance of the whole length of the resistance heater element tended to decrease, rather than to increase, as shown in, for example, FIG. 2.
It is an object of the invention to prevent increase of the temperature difference on the heating surfaces and occurrence of heat spots in a broad temperature range including a high temperature region in a heating apparatus comprising a substrate made of a ceramic material with a heating surface, a plurality of resistance-heating elements buried in said substance, pairs of terminals, each pair of the terminals being attached to a respective one of the resistance-heating elements to supply alternating current there to, AC power sources each connected to respective one of said pairs of the terminals for the respective resistance-heating elements to supply the alternating current thereto.